That's quite a varied trip, Sharon, so appreciate it might be hard to pack for! My advice in a word? Layers. Pack lots of T-shirts, a few long-sleeve shirts, a light rain jacket and sweater. You should also pack a pair of convertible pants for dual-use if you're out hiking or sightseeing and need an easy way to cool things off. Santiago, which is at elevation, is liable to be quite mild, though the city can get quite smoggy, especially in summer. As you head around from Chile around the tip to Ushuaia, know that this city has something of a reputation as a shopping outpost, so maybe a good opportunity to ditch some of the smellier gear in place of a few fresher duds? I expect it'll be a little cooler and wetter down there, so again a light rain jacket would be handy. Another piece of gear by which some travelers swear? Wool. Namely, merino wool T-shirts, jerseys, and socks that are lightweight, breathable, and odor-free. Check out the stuff from people including Icebreaker, Ibex, and SmartWool. Patagonia also has a line of travel gear. One thing worth thinking about with wool is that it'll come in handy once you hit the stylish enclaves of B.A., where it can do double-duty as an accessory during your late-night tapas and dancing forays!